The present disclosure relates generally to integrated circuit (IC) design using low voltage metal-oxide-semiconductor (MOS) transistors (MOSFETs), and more particularly, to a method for protecting the core circuitry of an integrated circuit (IC) from damage that may be caused by high voltage electrostatic discharge (ESD).
By using MOSFETs in an ESD protection circuit design, it can improve the manufacturing quality of the product. An invention that uses MOSFETs requires only the monitoring of electrical characteristics of MOS, diodes, and resistance. Most commercial MOS foundry already monitors these electrical characteristics during production. Other inventions using components such as bipolar transistors BJT, parasitic BJT of MOSFETs, and silicon-controlled rectifier SCRs require more monitoring. If an ESD circuit uses devices that the foundry does not monitor, the ESD performance cannot be controlled since the electrical characteristics of these devices may fail.
Electrostatic discharge may enter an IC through bond pads, which are connections from outside circuitry to the IC. They are usually used for supplying electric power, electric ground, and electric signals. These electrostatic discharges may be created in many different ways. For example, when parts of an external pad leading to the IC are touched by a person, he or she can create static electricity strong enough to destroy circuitry of an IC. In a MOS transistor, the gate oxide is most susceptible to damage. A voltage slightly higher than the supply voltage can destroy the gate oxide of the transistor. ESD created by common environmental sources can carry up to tens of thousands of volts when it occurs. Such voltages can damage the circuitry even though the charge and any resulting current are extremely small. To avoid these damaging voltages from building up, it is important to discharge any static electricity at the moment of occurrence. In order to protect the IC from ESD, implementation of protection circuits are necessary.
An ESD protection circuitry needs to allow the IC to operate normally while providing protection for the IC during ESD occurrences. ESD protection circuitry is typically implemented to ICs at the bond pads. The protection circuit can isolate itself from normal operation of the IC by blocking current from flowing through itself. During operation of an IC, electric power is supplied to VDD pad, and electric ground is supplied to a VSS pad. Many other pads are assigned to carry electronic signals that are supplied from outside or generated from IC. When the IC is unconnected, all pads are grounded to zero voltage.
In order for a protection circuit to work correctly, ESD will need to act as a brief power supply for one or more pads in an isolated IC, while the other pads remain floating, or grounded. Because the other pads are grounded, when ESD acts as a power supply at a randomly selected pad, the protection circuitry acts differently than it does when the IC is operating normally. When an ESD event occurs, the protection circuitry must quickly become current conductive so that the electrostatic charge is conducted to VSS ground; and, thus, dissipated before damaging voltage builds up.
With the demands of smaller size and lower power consumption on today's technology, circuitry is also shrinking in size, and using lower voltage components in order to save power. Lower voltage sources are used in internal circuits of an IC to save room, power, and money. When it comes to dealing with high voltage ESD, there is a need to solve the issue by using only low voltage components.
In the case of a high voltage ESD protection circuit, high voltage MOSFETs are usually used since low voltage MOSFETs have a thinner gate oxide which is vulnerable when high voltage is applied. The difference between low voltage and high voltage MOSFETs is the thickness of the oxide gate. With low voltage MOSFETs, the operating voltage can be lower than regular high voltage MOSFETs. Due to the thick layer of oxide gate, the high voltage oxide gate takes up more space and cost.
As such, desirable in the art of IC designs are additional designs using low voltage MOSFETs to provide the same kind of ESD protection.